Characterization and DFN Modelling of the Fracture Network in a Mesozoic Karst Reservoir: Gomba Oilfield, Paleogene Basin, Central Hungary

Abstract

Reservoir rocks at Gomba oilfield, located in the Hungarian Paleogene Basin, include fractured and karstified Triassic carbonates with significant fluid storage potential. However little information is available about the fracture network in these carbonates which has led to production problems at Gomba. The purpose of this study is to investigate the relationship between the spatial distribution of high porosity zones in the carbonates and the microfracture system. For this evaluation, individual fractures were studied and used in a discrete fracture network (DFN) modelling exercise at two wells, Gomba‐1 and Gomba‐3. The investigation took place at two different scales. Fracture length and aperture distributions were derived at the micro‐scale (mm to cm), whereas fracture density and fracture orientation data were investigated at the reservoir scale. The results of both investigations were taken into consideration in the modelling. The results of the fracture network models were compared to reservoir lithologies which ranged from fractured carbonates to collapse breccias. Based on the results, it appears that the porosity associated with a significant proportion of the fractures was increased as a result of dissolution. Fracture distribution was not uniform along the studied well paths, and fracture orientation was chaotic at particular depth intervals. On the basis of the DFN models, three different fractured zones are predicted to occur in the reservoir, but only two of the zones have significant fracture porosity. Comparing these results with petrographic observations, an epigene karst phase is proposed which is older than (or the same age as) the time of hydrocarbon migration; karst‐related voids were therefore important pathways for fluid migration. The void system and karst caverns partly collapsed during subsequent burial, resulting in karst‐related traps. By analogy with modern cave systems, the karst cavern zones at Gomba probably extend horizontally rather than vertically, and are oriented NE‐SW parallel to major structural lineaments.

Review of Modeling Approaches to Groundwater Flow in Deformed Carbonate Aquifers

“…The coupling of a DFN with a CN approach can also have an impact on modeling flow at much higher depths (∼1-3 km) in mediumand high-enthalpy geothermal reservoirs. Paleokarst features in bedding plane discontinuities and sub-vertical joints were detected at these greater depths significantly influencing the recovery of production wells (Bauer and Tóth 2017). A DFN must be coupled to a CN to represent fluid flow in marine carbonate rocks that have been exposed to episodes of emersion and erosion, and hence to the development of paleokarst cavities (Vecsei and Sanders 1997;Brandano 2017).…”

Localisation of Ancient Migration Pathways inside a Fractured Metamorphic Hydrocarbon Reservoir in South-East Hungary

“…As a result, interconnected fracture sub-systems as well as their spatial positions were determined for all 10 models. Further details on the mathematical background of the simulation procedure have been discussed in many previous papers [32][33][34][35].…”

Results, problems and future tasks of palaeostress and fault-slip analyses in the Pannonian Basin: the Hungarian contribution

“…This topic leads already to a major challenge, to under standing the role of fractures in fluid flow, basin-wide and locally. From the "fluid side" several results are embedded in publications (HAVRIL et al 2016(HAVRIL et al , 2018 and in divers applied researches like those conducted at geothermal fields and reconstruction of fracture network; its connection to fluid flow and mineralisation is under way in several parts of the Pannonian Basin (DABI et al 2013, BAUER & M. TÓTH 2017, M. TÓTH 2018. Although this is not strictly the topic of fault-slip analysis, but potentially, this will be the field where hydrogeology, hydrocarbon geology and brittle structural geology would (should) meet.…”